Method and device for diagnosis of the refrigerant quantity in an air conditioning system

ABSTRACT

A method for diagnosis of the refrigerant quantity in an air conditioning system in which an evaporator (10) followed by a liquid-separating accumulator (11), a compressor (12), a condensor (13), and a shut-off device (14) are connected in series after one another in a closed circuit for circulation of a refrigerant. The pressure of the refrigerant is measured between the condensor (13) and the shut-off device (14). The temperature of the refrigerant is measured after the compressor (12) and also before the evaporator (10). The measured values are compared with stored set values, for determining whether the refrigerant quantity is within, or outside, a reference interval.

TECHNICAL FIELD

The present invention relates to a method and a device for diagnosis ofthe refrigerant quantity in an air conditioning system, in which anevaporator followed by a liquid-separating accumulator, a compressor, acondensor, and a shut-off device are connected in series after oneanother in a closed circuit for circulation of a refrigerant.

BACKGROUND OF THE INVENTION

Air conditioning systems in vehicles must normally be able to functionin greatly varying conditions, for instance regarding ambienttemperature and atmospheric humidity. Also, its function depends on thequantity of refrigerant in the closed system.

The most common cause of malfunctions in air conditioning systems is atoo high or too low refrigerant quantity. A small leakage in an airconditioning system can be very difficult to discover. If thecompressor, over a very long period of time, is forced to function witha too low refrigerant quantity, it might break down.

Even a slight shortage of refrigerant can lead to the following negativeconsequences:

low undercooling after the condensor,

high overheating after the evaporator,

the evaporation pressure is lower than normal, and

the output temperature of the compressor is higher than normal.

An excess of refrigerant might, on the other hand, lead to the followingnegative consequences:

high undercooling after the condensor,

the condensing pressure is higher than normal,

the output temperature of the compressor is lower than normal,

the evaporation pressure is higher than normal, and

the overheating after the evaporator is low.

In practice, a shortage as well as an excess of the refrigerant willforce the compressor to work more often and during longer time periodsthan what otherwise would have been necessary.

Up to now, no suitable devices have existed for measuring therefrigerant quantity in a closed system with capillary tubes as ashut-off means, and with a liquid-separating accumulator after theevaporator. For this reason, service and maintenance of cars equippedwith air conditioning systems usually mean that the liquid must beevacuated from the system when measuring the quantity of therefrigerant. This evacuation is time consuming, since the systemcomprises rather long fluid lines with narrow passages. Draining andfilling of the refrigerant also lead to risks of discharging substanceswhich are harmful to the environment.

U.S. Pat. No. 4,757,693 describes an air conditioning system with anelectronic circuit for detection of malfunctions, for example due to aninaccurate refrigerant quantity. This air conditioning system is howeverequipped with a tank for the refrigerant between the condensor and theexpansion valve. U.S. 4,308,725 describes a detector for indicating thelevel of the refrigerant in such a storage tank.

SUMMARY OF THE INVENTION

Consequently, a purpose of the present invention is to provide a methodand a device for a quick and simple diagnosis of the refrigerantquantity in an air conditioning system which does not have a tank forthe refrigerant.

This object is solved in the method according to the invention in thatthe pressure of the refrigerant is measured between the condensor andthe shut-off device, that the temperature of the refrigerant is measuredafter the compressor and also before the evaporator, and that themeasured values are compared with stored set values for determiningwhether the refrigerant quantity is within, or outside, a referenceinterval. The device according to the invention is characterized bypressure sensing means for detecting the refrigerant pressure betweenthe condensor and the shut-off device, temperature sensing means fordetecting the refrigerant temperature after the compressor and beforethe evaporator respectively, and by an analysis unit which is arrangedto compare input data from the pressure sensing and the temperaturesensing means with stored set values.

Advantageous embodiments of the invention will be apparent from thesubsequent dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described in greater detailwith reference to the accompanying drawings, wherein

FIG. 1 schematically shows an air conditioning system with a diagnosissystem, according to a first embodiment of the invention,

FIG. 2 shows in an analogous way a diagnosis system according to asecond embodiment of the invention,

FIG. 3 shows a diagnosis system suitable for use in workshops, and

FIG. 4 is a diagram which shows the quantity of refrigerant in a systemas a function of the ambient temperature and the atmospheric humidity.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The air conditioning system shown in FIGS. 1 and 2 comprises anevaporator 10 with a following liquid-separating accumulator 11, acompressor 12, a condensor 13 and a shut-off device 14 which areconnected in series after one another in a closed circuit for thecirculation of a refrigerant.

According to a first embodiment of the invention which is shown in FIG.1, a separate monitoring unit 15 is connected to the air conditioningsystem by means of a control relay 16 for controlling the compressor 12,and also through a pressure-sensing means 17 for detection of therefrigerant pressure P₁₇ between the condensor 13 and the shut-offdevice 14. Furthermore, the monitoring unit is provided withtemperature-sensing means 18, 19 for detection of the refrigeranttemperature T₁₈ after the compressor 12 and the refrigerant temperaturebefore the evaporator 10, respectively.

Measurements from the pressure sensor 17 are used for derivingcorresponding temperature values T_(P17). This correspondence is basedon known thermodynamic relationships providing the temperature values asa function of the condenser pressure. A database, i.e. a tablecomprising a set of such corresponding temperature pressure values, isstored in the monitoring unit 15 for providing this conversionoperation.

The monitoring unit 15 controls the half-speed mode and the full-speedmode of the compartment fan by means of a dual relay 20. In addition tothis, the unit 15 receives input signals regarding the operation of theengine via an engine speed sensor 21, regarding the on/off-condition viaa sensor 22, and information regarding the compartment fan by a sensor23.

The unit 15 is arranged to compare input data from the pressure-sensingand temperature-sensing means 17-19 with stored set values, wherebydeviations from these values will result in a warning indication bymeans of a warning light 24 on the dashboard of the vehicle.

In the preferred embodiment, the above-mentioned temperature andpressure values are used to calculate a numerical value for use in thiscomparison, according to the following equation:

    (T.sub.18 -T.sub.19)/(T.sub.P17 -T.sub.19)

in which T₁₈ is the temperature measured by the first temperature sensor18, T₁₉ is the temperature measured by the second temperature sensor 19and T_(P17) is the temperature corresponding to the pressure valuemeasured by the pressure sensor 17. This equation gives a relativelycomplete picture of the overall conditions of the entire airconditioning system, i.e. both upstream and downstream of the condenser13. Also, it provides measurements which are essentially independent ofexternal influence. Other equations taking account of the pressure andtemperature measurements could, of course, also be used.

FIG. 2 shows an alternative embodiment of the invention in which amonitoring unit 25 is integrated with an automatic air conditioningsystem for performing periodical control of the compressor 12 andautomatic shut-off at a low refrigerant quantity, in the same manner asthe previously described embodiment. Functionally, this embodimentdiffers from the previous one in that information regarding the ambienttemperature and the engine temperature is available and may be used fora timer-controlled start of the compressor as well as for a moreaccurate diagnosis of the switched-off state.

FIG. 3 shows a diagnosis unit 27 intented for use in a workshop,comprising a T-connection 28 which is mounted between the ordinaryconnection of the vehicle's cabling 29 and the diagnosis unit 15; 25. Atemperature sensor 30 is manually fitted at a suitable location betweenthe evaporator 10 and the accumulator 11. The value of the ambienttemperature is entered in the diagnosis instrument 31. The diagnosis ismade while idling and with the compartment fan at full speed.

From the diagram shown in FIG. 4, it is apparent how the rise intemperature of the compressor in relation to the rise in pressure of thecompressor depends on the refrigerant quantity. This relationship can beexpressed as a function of the difference between the condensingtemperature and the evaporator temperature. The Y-axis of the diagram isbased on the temperature after the compressor, the temperature beforethe evaporator and the difference between the saturation temperaturecorresponding to the pressure after the condenser and the temperaturebefore the evaporator. Specifically, the Y-axis represents values of thefunction (T₁₈ -T₁₉)/(T_(P17) -T₁₉), and the X-axis representsexperimentally measured refrigerant quantity, in grams, at variousvalues of that function. Four graphs display various operationconditions with respect to the ambient temperature and the atmospherichumidity.

The monitoring unit 15 determines refrigerant quantity from values ofT_(P17), T₁₈ and T₁₉ by using a table of predetermined valuesrepresenting the graphs of FIG. 4 stored in memory. A value of

    (T.sub.18 -T.sub.19)/(T.sub.P17 -T.sub.19)

is first calculated from the measured values, and a refrigerant quantityis then determined using the stored table. In accordance with that data,a specific value of the refrigerant quantity can thus be derived byfollowing a graph corresponding to the particular operating conditions.For example, a value of 2 which is calculated by means of said equationcorresponds to a refrigerant quantity of approximately 520 grams(assuming a temperature of 50°), as shown in FIG. 4.

Underfilling leads to increasing overheating, causing an increased inlettemperature to the compressor and thereby also an increased outputtemperature.

If overfilled, an overboiling in the evaporator occurs, which leads todrops of liquid entering the compressor. This results in a decrease ofthe output temperature due to the cooling effect of the liquid.

The invention is not limited to the abovementioned embodiments, butother variations are also possible within the scope of the followingclaims.

I claim:
 1. A method for determining the quantity of refrigerant in anair conditioning system comprising a closed circuit for circulating saidrefrigerant sequentially through an evaporator, a liquid-separatingaccumulator, a compressor, a condenser, and shut-off means, said methodincluding measuring the pressure of said refrigerant between saidcondenser and said shut-off means, measuring the temperature of saidrefrigerant downstream of said compressor, measuring the temperature ofsaid refrigerant upstream of said evaporator, and comparing saidmeasured values of said pressure and temperature of said refrigerantwith predetermined values therefor in order to determine whether saidquantity of said refrigerant falls within a predetermined referenceinterval.
 2. Apparatus for determining the quantity of refrigerant in anair conditioning system comprising a closed circuit for circulating saidrefrigerant sequentially through an evaporator, a liquid-separatingaccumulator, a compressor, a condenser, and shut-off means, saidapparatus including pressure sensing means for determining the pressureof said refrigerant between said condenser and said shut-off means,temperature sensing means for measuring the temperature of saidrefrigerant downstream of said compressor, temperature sensing means formeasuring the temperature of said refrigerant and upstream of saidevaporator, and analyzing means for comparing said measured values ofsaid pressure and temperature of said refrigerant with predeterminedvalues therefor in order to determine whether said quantity of saidrefrigerant falls within a predetermined reference interval.
 3. Theapparatus of claim 2 wherein said analyzing means is integrated with anautomatic air conditioning system.
 4. A method for determining thequantity of refrigerant in an air conditioning system comprising aclosed circuit for circulating said refrigerant sequentially through anevaporator, a liquid-separating accumulator, a compressor, a condenser,and shut-off means, said method includingmeasuring the pressure of saidrefrigerant between said condenser and said shut-off means, measuringthe temperature of said refrigerant downstream of said compressor,measuring the temperature of said refrigerant upstream of saidevaporator, computing the difference between said temperature of saidrefrigerant downstream of said compressor and said temperature of saidrefrigerant upstream of said evaporator divided by the differencebetween a temperature of said refrigerant corresponding to said pressureof said refrigerant between said condenser and said shut-off means, andsaid temperature of said refrigerant upstream of said evaporator,deriving a value from a result of said computing step, said valuecorresponding to an actual refrigerant quantity, and comparing saidvalue with a predetermined value in order to determine whether saidquantity of said refrigerant falls within a predetermined referenceinterval.